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    <script>
        // 求 1 * 2 * 3 * 4 * 5
        function fn (n) {
            if (n === 1) {
                return 1;
            }
            return n * fn( n - 1);
        }
        console.log(fn(3));
        // 思路：假如用户输入的是3
        // return 3 * fn (2);
        // return 3 * ( 2 * fn (1));
        // return 3 * ( 2 * 1);
        // return 3 * (2);
        // return 6;
    </script>
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